Projection exposure systems for printing circuit patterns such as semiconductor devices drawn on masks onto wafers are requested to fulfill the performance of printing fine patterns with high resolution. In general, the greater the numerical aperture (NA) of the exposure lens becomes or the shorter the wavelength of exposure light becomes, the better the resolution becomes. Methods of making the NA large bring about lowering in depth of focus at the time of pattern printing, and hence there is a limit in magnitude of NA. Therefore, studies for improving resolution by using short wavelength beams such as X-rays have heretofore been made. As the wavelength becomes shorter, however, the beam is absorbed more easily. Therefore, it is difficult to realize an imaging optical system using a transmissive lens like conventional exposure systems using a mercury lamp as the light source. Therefore, methods using a reflective imaging optical system have been proposed.
Conventional reflective imaging optical systems premised on the assumption that X-rays are used are described in JP-A-63-18626 and JP-A-63-312638. In each of these conventional examples, configuration of an imaging optical system for printing mask patterns onto a wafer is studied in detail. As for the proximity exposure in which patterns are printed under the condition that the mask is in close vicinity to the wafer, an illumination optical system in which an exposure field is expanded by scanning a reflecting mirror is disclosed in JP-A-1-96600. Further, in JP-A-63-62231, introduction of a mirror for providing arc field illumination in printing mask patterns by using an imaging optical system having one or two reflecting mirrors is described.